1. Field of the Invention
The present invention relates to negative-working printing plates which can be exposed by UV, visible, or infrared radiation. In particular, the present invention relates to printing plates comprising polymeric binders containing polyethylene oxide segments.
2. Background of the Invention
Radiation-sensitive compositions are routinely used in the preparation of high-performance printing plate precursors. There are primarily two ways of improving the properties of radiation-sensitive compositions and thus also of the corresponding printing plate precursors. The first way addresses improvement of the properties of the radiation-sensitive components in the compositions (frequently negative diazo resins or photoinitiators). The other way deals with improvement of physical properties of the radiation-sensitive layers through the use of novel polymeric compounds (“binders”).
The latest developments in the field of printing plate precursors deal with radiation-sensitive compositions which can be imagewise exposed by means of lasers or laser diodes. This type of exposure does not require films as intermediate information carriers since lasers can be controlled by computers.
High-performance lasers or laser diodes which are used in commercially available image-setters emit light in the wave-length ranges of between 800 to 850 nm and between 1060 and 1120 nm, respectively. Therefore, printing plate precursors, or initiator systems contained therein, which are to be imagewise exposed by means of such image-setters have to be sensitive in the near IR range. Such printing plate precursors can then basically be handled under daylight conditions which significantly facilitates their production and processing.
There are two possible ways of using radiation-sensitive compositions for the preparation of printing plates. For negative printing plates, radiation-sensitive compositions are used wherein after an imagewise exposure the exposed areas are cured. In the developing step, only the unexposed areas are removed from the substrate. For positive printing plates, radiation-sensitive compositions are used whose exposed areas dissolve faster in a given developing agent than the non-exposed areas. This process is referred to as photo-solubilization.
Negative-working plates typically require after imagewise exposure a preheating step, as described for example in EP 0 672 544, EP 0 672 954 as well as U.S. Pat. No. 5,491,046 and EP 0 819 985. These plates require a preheating step within a very narrow temperature range which only causes a partial crosslinking of the image layer. To meet current standards regarding the number of printable copies and the resistance to press room chemicals, an additional heating step—referred to as a post bake step—is carried out during which the image layer is crosslinked further.
U.S. Pat. No. 4,997,745 describes photosensitive compositions comprising a dye absorbing between 300 and 900 nm and a trihalomethyl-s-triazine compound.
In U.S. Pat. No. 5,496,903 and DE 196 48 313, photosensitive compositions are described which in addition to a dye absorbing in the IR range comprise borate co-initiators; also, halogenated s-triazines are described as further co-initiators.
Further photopolymerizable compositions with initiator systems are described in U.S. Pat. No. 5,756,258, U.S. Pat. No. 5,545,676, U.S. Pat. No. 5,914,215, JP 11-038633, JP 09-034110, U.S. Pat. No. 5,763,134 and EP 0 522 175.
U.S. Pat. No. 6,245,486 discloses radiation sensitive printing plates, including on-press developable plates. However, this patent requires compositions having an IR ablatable mask layer over a UV addressable, negative-working, on press developable, free radical polymerizable layer.
U.S. Pat. No. 6,245,481 discloses IR-ablatable, UV-photopolymerizable two-layer compositions that require IR exposure followed by UV flood irradiation.
U.S. Pat. No. 5,599,650 discloses UV addressable, negative-working, on press developable printing plates based on free radical polymerization. This patent requires a free radical quencher polymer, specifically one containing nitroxide groups, in an overcoat layer to facilitate developability.
U.S. Pat. No. 6,071,675 discloses similar printing plates to U.S. Pat. No. 5,599,650 but additionally requires adding dispersed solid particles to the imaging layer to improve on-press developability or to reduce tackiness.
U.S. Pat. No. 6,309,792 and WO 00/48836 describe IR-sensitive compositions comprising a polymeric binder, a free radically polymerizable system, and a specific initiator system. The compositions of WO 00/48836 require a preheat step after the exposure for sufficient hardening of the compositions. The printing plate precursors must be developed with an aqueous developer.
U.S. Pat. No. 6,864,040 describes IR sensitive compositions containing leuco dyes additional to those described in U.S. Pat. No. 6,309,792 and WO 00/48836. U.S. Pat. No. 6,864,040 requires a preheat step after IR exposure and an aqueous development step for processing.
U.S. Pat. No. 5,204,222 teaches a composition comprising polymerizable ingredients in conjunction with a polymer binder comprising a polyurethane main chain. The side chains of the polymer binder do not comprise a polyethylene oxide chain.
U.S. Pat. No. 5,800,965 teaches a composition, suitable for flexographic plates, comprising monomers of polyethylene glycol as polymerizable components.
U.S. Pat. No. 6,037,102, also directed to flexographic plates, teaches a photopolymerizable composition comprising a graft copolymer having polyvinyl alcohol grafts on a polyethylene oxide (PEO) main chain polymer.
EP 1 117 005 discloses photopolymerizable compounds which contain polyethylene oxide chains having 1-10 ethylene oxide units. The invention is exemplified by the use of polymers having one ethylene oxide unit. With more than ten ethylene oxide units, both resolution and water resistance of cured products decrease. Binder resins having sufficiently long PEO segments in accordance with the present invention are not disclosed.
U.S. Pat. No. 6,582,882 discloses graft copolymers comprising polyethylene oxide side chains, but does not teach a composition comprising polymerizable components or initiators. The side chains may further comprise a hydrophobic segment between the polyethylene oxide segment and the main chain, and a hydrophobic segment at the terminus of the polyethylene oxide side chains.
U.S. Pat. No. 6,846,614 discloses polyalkylene ether polymers and copolymers, including block copolymers of polyethylene oxide and polypropylene oxide. However, the polyalkylene ether polymers and copolymers disclosed in this patent do not provide sufficient differentiation for developability of the unexposed areas and durability of the exposed image areas.
None of the above patents or patent applications disclose polymerizable compositions which contain binder resins having PEO segments in accordance with the present invention.
The present invention therefore satisfies the need in the art for a printing plate and process for preparing a printing plate that does not require a preheat step or a development step. As a result of substantial studies, it was found that polymerizable compositions, which contain certain polymeric binders having polyethylene oxide (PEO) segments, are readily developable in aqueous developers, including on-press developability with fountain solution and printing ink. Furthermore, following imagewise exposure to electromagnetic radiation in the ultraviolet, visible or infrared spectral regions, the exposed regions resist developability and serve as durable, ink receptive image areas, without the need for a predevelopment heating step. Thus, it was found that certain polymeric binders having PEO segments, surprisingly, enhance differentiation of the exposed and unexposed areas by facilitating developability of the unexposed areas together with enhancing durability of the exposed image areas.